This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which transmit different ultrasonic frequencies for ultrasonic sum and difference frequency imaging.
Harmonic imaging is in widespread use in ultrasonic imaging today because of the advantages in signal segmentation and image clarity which it provides. Harmonic imaging at present has two applications: harmonic contrast imaging and tissue harmonic imaging, both of which involve nonlinear signal components. Examples of these two types of harmonic imaging may be found in U.S. Pat. Nos. 5,833,613 (for harmonic contrast) and 5,879,303 (tissue harmonic) of which I am a co-inventor. Harmonic contrast imaging provides the advantage of sharp signal segmentation and signal-to-clutter improvement due to the fact that harmonic contrast agents return relatively strong harmonic signals in response to fundamental frequency insonification. These relatively strong harmonic contrast signals are readily distinguished from the fundamental frequency and relatively low level harmonic signals returned from tissue and other substances in the body. Tissue harmonic imaging, while having a signal-to-noise deficit as compared to fundamental frequency imaging, provides an advantage in image clarity through a reduction in image clutter. Since the distortion of acoustic waves passing through tissue which gives rise to the harmonic components only begins to develop and build as the waves travel deeper into the body, near field scatterers which are a source of image clutter can scatter only the low or insignificant levels of harmonic energy present at shallow depths. Thus, tissue harmonic images will exhibit reduced clutter as compared to fundamental frequency images, although at reduced signal levels due to the lower level of the harmonic components and by reason of depth dependent attenuation of the higher frequency harmonic signals. It would be desirable to utilize nonlinear signals which afford the foregoing advantages but with greater signal levels, better signal-to-clutter ratios, broader bandwidths, and reduced depth dependent attenuation.
In accordance with the principles of the present invention, ultrasonic imaging is performed by transmitting an ultrasound beam with two or more different frequency components. When the beam passes through a nonlinear transmission medium or encounters a nonlinear scatterer the different frequency components intermodulate and develop sum and difference frequencies, which are contained in returning echo signals, detected and used to form an ultrasonic image. In accordance with one aspect of the present invention, a multiple frequency beam transmitter is provided with multiple digital transmit signal generators producing signals of different frequencies which are combined digitally to form a composite transmit signal. In accordance with another aspect of the present invention, composite multiple frequency transmit signals are digitally stored, then selected and converted to analog form for transmission. In accordance with a further aspect of the present invention, the transmitted frequency components are located on opposite sides of the transducer peak response (center) frequency, with a received difference frequency signal located in the vicinity of the peak response point of the transducer characteristic. In accordance with another aspect of the present invention, the amplitudes of the transmitted frequencies are chosen in consideration of the effects of depth dependent attenuation and/or the properties of contrast agent microbubbles.